This invention relates to alloys for use in casting dental appliances. More particularly, it relates to chromium-containing alloys and a process for producing same in a form suitable for use in casting dental appliances and capable of being processed, prior to casting, to wrought form by hot working.
Gold alloys have traditionally been the material of choice among dental laboratories for use in making cast restorations such as denture bases, partial dentures, crowns and bridgework. These alloys are usually formulated by dental gold manufacturers in accordance with American Dental Association specifications and supplied to dental laboratories in the form of rolled strip cut to exact weight pieces for remelting and casting into dental appliances using heated molds.
Over the years, as a result of rising gold prices, base metal alloys have been sought after as a substitute for dental gold. An early example of such non-noble metal alloys are the cobalt/chromium alloys, introduced in 1933. These alloys, and others developed more recently, exhibit resistance to corrosion in the mouth comparable to that possessed by gold alloys. For example, according to J. R. Lane, A Survey of Dental Alloys, J. Am. Dental Assn., 39, 414-437 (1949), this corrosion resistance is due in large part to the presence of chromium in the base metal alloy, a chromium content in excess of 18 percent by weight being generally effective for this purpose. Unlike dental gold, however, cobalt/chromium alloys are undesirably hard and exhibit low ductility. Thus, undue hardness makes finishing (e.g., grinding and polishing) difficult and expensive, while low ductility complicates adjustment of the finished appliance by the dentist. Another unattractive feature of these alloys is their higher melting temperatures compared to gold alloys. Furthermore, cobalt/chromium alloys cannot be easily wrought or rolled into small weight pieces as can the gold alloys. Instead, they are supplied as small cast ingots for remelting and casting into dental appliances. However, the use of wrought alloy rather than cast ingots by dental laboratories is desirable because the former possesses a number of advantages. Thus, the use of wrought alloys for casting in heated molds results in cast dental appliances of sufficient ductility to permit adjustment by dentists and dental technicians. Wrought alloy can be made in larger melts under controlled conditions which provides greater melt-to-melt uniformity. Additionally, wrought material permits production of the small shaped pieces by mechanical means, which in turn permits cost reduction.
Although cobalt/chromium alloys have over the years established a secure position in the dental appliance field, their use has been generally restricted to the casting of relatively large appliances, primarily because of the aforesaid drawbacks (low ductility and high finishing cost) in the properties of these alloys. However, the recent dollar devaluations and establishment of the two-tier gold price system have intensified the need for base metal alloys suitable for all types of dental castings, both large and small.
Among the newer base metal alloys showing promise in the dental field are the iron/chromium, iron/nickel/chromium, and nickel/chromium alloys. These newer chrome alloys, like the older cobalt/chromium alloys, are stronger, lighter in weight, more resistant to staining and, of course, less expensive than the gold alloys. Although the higher melting temperatures of the newer base metal alloys are a disadvantage insofar as the production casting of dental appliances is concerned, their major drawback resides in the sensitivity of these alloys with regard to ductility and workability to the presence of even miniscule amounts of so-called "tramp" elements, namely, carbon, oxygen and nitrogen. The problem is particularly irksome in dentistry because the aforesaid impurities are difficult to control in the small foundry melts of ingots used in casting appliances. According to Skinner and Phillips, The Science of Dental Materials, page 583 (W. B. Saunders Co. 1967), the control of carbon content is most critical, since small variations thereof can have a pronounced effect on the strength, hardness and ductility of the alloy. Carbon can react with the metallic components of the alloy to form carbides. These carbon-metal compounds can dramatically alter the alloy's useful properties of strength and ductility as well as increase the difficulty of finishing the cast appliance.
The control of carbon in chromium/base metal alloys is exceedingly difficult during both the manufacture of ingots and their use by dental laboratories in casting procedures. This is also true when a flame is used during casting whereupon an undesirable amount of carbon is usually incorporated into the alloy. Even assuming that the carbon content of the alloy can be controlled satisfactorily, the additional problem of contamination during the melting and casting stages with oxygen and nitrogen from the atmosphere must be dealt with. As a result, the production of base metal dental alloys has in practice been restricted to the casting of unwrought, small ingots weighing about 5-grams following the pattern set by the earlier cobalt/chromium alloys notwithstanding the inherent capability of the newer alloys to be hot-worked to form wrought pieces. This fact has severely limited the economic benefits which would otherwise be realized in replacing gold altogether in the dental field. Heretofore, efforts at overcoming the aforementioned obstacles have been directed primarily at introducing compositional variations in existing chrome alloys, for example, by the addition of special elements or agents. The work described in K. Asgar and B. O. Techow, A New Alloy for Partial Dentures, J. Prosthetic Dentistry, 23(1), 36-43 (January, 1970) typifies these efforts. However, for a variety of reasons, the above-described problems have yet to be solved in a commercially acceptable manner through the introduction of new alloys. A need therefore exists, not for a new alloy, but rather, for an economical process whereby conventional chrome alloys can be cast and hot-worked into small wrought pieces of quality comparable to that presently enjoyed by gold.
Accordingly, it is an object of the present invention to provide a process for making alloys in a wrought form suitable for remelting and casting into dental appliances of adequate strength and ductility.
Another object is to provide a process for the production casting of chrome alloys from wrought pieces into dental appliances which are free from deleterious amounts of tramp elements such as carbon, nitrogen and oxygen.
Another object is to provide chrome alloy wrought pieces which are initially free of deleterious amounts of tramp elements and which can be used in the casting of both large and small dental appliances without contamination by deleterious impurities.
These and other objects of the invention, as well as a fuller understanding thereof can be had by reference to the following detailed description, drawing, and claims.